The present invention relates generally to optical laser telescopes, and more specifically to a technique and apparatus for optically phasing an array of multiple telescopes for use as a laser transmitter.
A synthetic aperture is formed when separate optical systems are combined to function as a single larger aperture. When an aperture is synthesized, independent optical systems are phased to form a common image field with resolution determined by the maximum dimension of the array and therefore exceeding that produced by any single element. By optically phasing an array of multiple telescopes, a synthetic aperture is formed which can achieve the performance of an equivalent sized, single laser transmitter.
Phased arrays are currently in use in radar systems. The successful application of phasing an array of multiple telescopes into a synthetic aperture extends the numerous benefits of using arrays, as experienced by radar systems, to optical laser telescopes.
Phased arrays are modular. They can be built in stages and to some extent be operational as soon as the first telescope is operational. An array of independent telescopes has functional flexibility. Several simultaneous operations can be carried out by individual telescopes within a synthetic aperture. For example, images can be directed to different cameras or spectrographic devices for simultaneous observations in separate imaging modes. When operated as a transmitter, a synthetic aperture has the option of sending beams in different directions.
Phased array apertures have virtually no size limitations. By modularly combining telescopes in a phased configuration, laser transmitters of previously unimaginable sizes can be constructed. Large optics fabrication has historically posed an impermeable barrier to building large aperture telescope systems. By phasing a number of reasonably-sized telescopes, extremely large transmitting apertures ca be achieved with present fabrication technology.
The optical phasing of separate transmitted beams, can be achieved by maintaining matched optical paths, when the laser transmitter is a system which provides inputs into multiple telescopes by dividing a single beam.
Techniques for achieving and maintaining matched optical paths fall into four general categories:
(1) Structural or optical metering trusses; PA1 (2) local loop phasing; PA1 (3) target loop phasing; and PA1 (4) hybrids or combinations of the above techniques.
The structural metering truss requires thermal control to maintain its integrity. For large systems operating at infrared or optical wavelengths, position monitoring of the structure is also required. An optical metering truss can be formed by a device such as a fan beam. Tolerances associated with the elements that produce an optical truss are a major disadvantage to the approach.
Target loops insure that an array is phased in the far field. The target may very well be a star in which case white light interferometry can be employed. Difficulties include possible low signal to noise ratios and a requirement for aperture sharing elements. Because focal arrays have an inherently limited phased field of view, phasing on targets introduces the prospect of an out-of-phase observation plane. Also, when phasing on nearby fast-moving targets, Doppler effects must be taken into account.
The present invention uses local loop phasing to control the phase of separately transmitted beams by adjusting the optical path lengths of the beam. Local loop phasing is an indirect measurement of the quantity of interest. Typically, a single source is injected into the system and divided to traverse the separate paths of all telescopes to be phased. Beams are recombined at a common plane for phase monitoring and control. White light interferometry with a Koester prism is a good example of local loop phasing. This technique is quite suitable for an imaging synthetic aperture. A transmitter, however, in this configuration is limited to a single source or a combination of local loop phasing and a separate operation for phasing multiple sources. Furthermore, beam injection requires an aperture sharing element which has fabrication drawbacks.
The optical phasing of separate transmitted beams of laser transmitters with monochromatic light sources is achieved by matching the optical paths. In systems which have polychromatic sources, the optical path lengths are adjusted.
Monochromatic light requires phasing only within a range of one wavelength. Polychromatic sources cannot tolerate 2.pi. ambiguities and therefore require both coarse and fine phase adjustments for multi-wavelength interference.
The present invention uses samples of the transmitted beams to control optical path lengths through the separate telescopes so that the beams add coherently at the receiver. The phasing concept is applicable both to systems which provide inputs to the multiple telescopes by dividing a single laser beam and to systems in which the inputs to the telescopes are multiple, phase-locked laser beams. The approach is also compatible with single line and multi-line lasers since all wavelengths are unambiguously phased. The application of the technique of the present invention extends the many benefits, described above, of using phased arrays, to optical laser telescopes. An excellent example of the current application of laser telescopes is contained in U.S. Pat. No. 4,295,741 issued to Gary E. Palma et al on Oct. 20, 1981, the disclosure of which is incorporated by reference. Palma et al disclose a laser transmitter system which achieves phase matching between a first and second laser beam emitted through first and second laser telescopes. While the disclosure of Palma et al is exemplary in the art, the laser source used therein is a single multi-line laser. The phasing of an array of separate and independent sources of multiple laser telescopes would result in the equivalent of a single laser transmitter, with all the advantages of phased arrays, as discussed above and as currently incorporated in radar technology.
In view of the foregoing discussion, it is apparent that there currently exists the need for an optical phase sensing and control system, which allows multiple independent optical telescopes to be used as a phased array in a laser transmitter. The present invention is directed towards satisfying that need.